1. Field of the Invention
The present invention relates to a radiographic imaging apparatus that performs imaging for radiation passing through a subject, and more particularly to control of an irradiation form of radiation.
2. Description of the Related Art
Hitherto, an apparatus, which obtains a radiographic image of a subject by irradiating the subject with radiation and detecting an intensity distribution of the radiation passing through the subject, has been widely used in the fields of industrial non-destructive inspection and medical diagnosis. A typical imaging method may be a film/screen method for radiation. The film/screen method is an imaging method which uses a photosensitive film, and a scintillator sensitive to radiation. The method requires chemical processing for development, and hence, it is difficult to perform real-time imaging.
In contrast, an imaging system using an image intensifier can perform real-time movie recording. For example, a mobile X-ray fluoroscopy apparatus 1 shown in FIG. 9 is known as an apparatus using an image intensifier. The X-ray fluoroscopy apparatus 1, which is provided with casters at a lower portion thereof, includes a carriage 2 and a horizontal shaft 3 provided above the carriage 2. The carriage 2 is horizontally movable by the casters. The horizontal shaft 3 can move horizontally as indicated by arrow Mh and vertically as indicated by arrow Mv, and rotate around a vertical supporting axis as indicated by arrow Rv. A C-shaped arm member 4 is fixed to a tip end of the horizontal shaft 3.
An X-ray source 5 and an image intensifier 6 are oppositely arranged at both tip ends of the arm member 4 so as to face each other. The arm member 4 is configured to rotate around a horizontal axis as indicated by arrow Rh in accordance with a movement of the horizontal shaft 3, and to rotate and move along a C-shaped locus as indicated by arrow Rc. The X-ray fluoroscopy apparatus 1 can align the X-ray source 5 and the image intensifier 6 at various positions with respect to a subject, by way of a plurality of moving mechanisms containing the movement of the horizontal shaft 3.
The X-ray source 5 outputs X-rays to a subject arranged inside the arm member 4. Then, the image intensifier 6 arranged opposite to the X-ray source 5 converts an X-ray image passing through the subject into an optical image. The optical image converted by the image intensifier 6 is optically condensed by an optical lens, and converted into an electrical signal by a TV camera. The electrical signal is reproduced as a visible image on a cathode-ray tube (CRT) or the like. The image information is A/D converted to be stored as a digital signal, processed with various image processing techniques to be useful information, and used in various diagnostic applications. Thus, a medical imaging diagnostic technology has progressed.
Meanwhile, in recent years, a semiconductor process technology has developed. In particular, a flat panel detector (FPD) which performs imaging for a radiographic image with a semiconductor sensor has been developed. FIG. 10 is a schematic illustration showing a radiographic system using such a FPD. A radiation source device 11 irradiates a subject P with radiation, and a radiographic imaging apparatus 13 containing a FPD 12 performs imaging for radiation passing through the subject P. The FPD 12 is a flat detector in which photoelectric conversion elements are arrayed on a flat substrate in a two-dimensional grating. The FPD 12 converts the radiation into visible light through a scintillator. The photoelectric conversion elements arrayed in the two-dimensional grating detect the visible light as an electrical signal. A controller 14 that controls the driving of the reading, image transferring, and the like, is connected to the radiographic imaging apparatus 13. The controller 14 performs digital image processing on an image output from the radiographic imaging apparatus 13, and allows a monitor 15 to immediately display a radiographic image of the subject P.
The image intensifier 6 of the related art has an X-ray incidence plane with a diameter of 6 to 12 inches. With regard to the optical condensing unit, the image intensifier 6 has a cylindrical shape being long toward the incidence plane. In some cases, the image intensifier 6 may not be installed at a desired position depending on the size. Thus, a reduction in thickness of the X-ray detector is demanded.
In contrast, the FPD 12 is a flat detector, and hence, an optical condensing system such as an image intensifier 6 is not necessary, thereby reducing the thickness of the X-ray detector. Also, an image would not be deformed at a peripheral portion, and an entire rectangular region can be effectively used.
In such an imaging system, a detecting panel is installed at a pedestal dedicated to an imaging mode for a standing position, a lying position, or the like. The detecting panel is selected as desired, and the imaging system is installed in and fixed to a radiation room. The imaging system is further reduced in weight, and is used instead of the image intensifier 6, in a mobile X-ray fluoroscopy apparatus, for example, as disclosed in Japanese Patent Laid-Open No. 2005-470.
When the X-ray detector is to be aligned with the subject, since the image intensifier of the related art has a symmetric cylindrical external shape, the X-ray detector need not be rotated within the X-ray incidence plane. To rotate an image, image processing is performed, or an optical system is rotated in the detector.
However, since the FPD has a rectangular shape because of a manufacturing process using a glass substrate and a circuit structure, the FPD must be aligned with the subject, and thus, is rotated within the plane.
In many cases, the X-ray imaging apparatus has an X-ray limiting mechanism to prevent emitted X-rays from leaking from the X-ray detector to the outside. In the case of a circular detector such as an image intensifier, even when the image intensifier is rotated, a beam limiting region does not exceed an imaging region as long as the image intensifier is rotated around the center.
In contrast, in the case of the rectangular detector, a beam limiting form 21 is also rectangular as shown in FIG. 11. Hence, even when a detector 22 is rotated within a plane around the center S of the detector 22, an angle of the detector 22 can be misaligned with an angle of the beam limiting form 21. In this case, a region 23 may appear in which X-rays leak to the rear side. Technically, it is possible to measure both angles and automatically align them with each other. However, such a configuration may increase the cost.